Add validation and data shuffle

[Nacho114]

Need to add data validation and shuffling to the data loader. (only inside each local client data, not between clients that is)

• [x] Update the return type of load to Promise<Data>
• [x] Add shuffle and seed mechanism (the TFJS version seemed to be broken, might be worth to test it again though)
• [ ] Add validation dataset, there is a validationSplit number param, but it does not seem to be supported on the current TFJS version we have. So either update TFJS and try it out, or do a split manually in the data loader, and add corresponding dataSplit param in builder.

[Nacho114]

To avoid things breaking it's best to be conservative for now, and just add the easier solution for validation set, and we can in a later point try to update tfjs

[Nacho114]

data shuffling is only done locally (not between clients)

[s314cy]

#338 indirectly depends on this

[martinjaggi]

did you guys check what's currently shown on the validation plot during training? if there is an issue we can remove it temporarily (discussed with hugo)

[s314cy]

#356 adds shuffling

[s314cy]

#372 adds validation split for image datasets

[martinjaggi]

done i guess in #356 and #499